Platelets play a major role in hemostasis in congenital defects in platelet function are associated with bleeding manifestations. The goal of this project is to define the biochemical mechanisms underlying the platelet dysfunction in patients with congenital platelet function defects and thereby enhance our understanding of the normal platelet activation mechanisms, which are vital to the development of newer therapeutic strategies for both bleeding and thrombotic disorders. The vast majority of patients with congenital platelet defects are generally characterized by impaired aggregation responses and dense granule secretion on activation; most of them have normal dense granule stores. These patients are lumped in the loosely defined group called "platelet secretion defects" or "activation defects". In them, the underlying biochemical and molecular mechanisms are totally unknown. Our hypothesis is that these patients have defects in the signal transduction mechanisms This project focuses on specific patients in whom we demonstrate hitherto undescribed deficiencies in two major proteins involved in signal transduction mechanisms, namely, a) phospholipase C-beta2 (Aim 1), and b) GTP- binding protein Galphaq (Aim 2), respectively. Our studies to date demonstrate that the coding sequence of PLC-beta2 and Galphaq gene is normal in these patients but the respective mRNA levels are decreased. We will study the transcriptional regulation of PLC-beta2 (Aim 1) and Galphaq (Aim 2) to define the mechanisms. In several other patient with platelet function defects, we have obtained evidence for abnormalities in the signaling events (e.g. Ca2+ mobilization, pleckstrin phosphorylation). In Aim 3, we will perform detailed studies in 3-4 patients and their family members to characterize the mechanisms leading to the impaired responses. This project represents application of state-of-the-art techniques to define the molecular mechanisms of platelet dysfunction in a group of patients who are very poorly characterized at present and constituted an untapped source of new information. These studies will provide new information on the role of two major protein, PLC-beta2 and Galphaq, in platelet signal transduction mechanisms.